The invention relates to a method of improving combustion in a combustion system having a circulating fluidized bed. It also relates to systems for implementing the method.
As is known, this type of system burns crushed solid fuels, liquid fuels, or gas fuels, in particular in fossil-fuel power stations. It suffers from the drawback of producing nitrogen oxides (NOx), and ways are thus sought of limiting the emission of such oxides, in particular by facilitating the release of the nitrogen-containing species in the fuel into an environment that is weakly-oxidizing or even reducing. Such limitation is conventionally obtained by designing hearths in which provision is made to enable the air intake to be staggered at different levels or xe2x80x9cstagesxe2x80x9d over the height of the hearth.
One known solution makes provision to inject xe2x80x9cprimaryxe2x80x9d air into a zone situated at the bottom of the hearth, at a flow rate that is determined so as to obtain an excess air coefficient such that a reducing atmosphere is generated therein. xe2x80x9cSecondaryxe2x80x9d air is injected on one or more stages above the bottom zone of the hearth so as to generate an oxidizing zone extending over the remainder of the height of the hearth.
Unfortunately, the known solutions do not provide sufficiently high performance, and an object of the invention is thus to make it possible to obtain a decrease in harmful nitrogen oxide (NOx) emissions in a combustion system having a circulating fluidized bed and associating, in known manner, a hearth and a cyclone, and to achieve said decrease without diminishing the combustion efficiency of the system.
The invention thus provides a method of improving combustion in a system of the circulating fluidized bed type, which system includes a hearth associated with at least one cyclone, in particular for delivering energy to a heat exchanger in an electricity-generating installation. The system is assumed to operate using fuel that is inserted into the bottom of the hearth, where a reducing atmosphere is created and where the fuel undergoes pyrolysis with separation into two phases, namely a solid phase made up of grains of coke and of nitrogen-coke coming from the original fuel, and a gaseous phase containing volatile matter and in particular the remaining nitrogen contained in the original fuel. The particles and the volatile matter rise up inside the hearth where combustion continues while leaving incompletely burnt particles which are sucked up and returned to the bottom of the hearth by cyclone action. The solid particles still containing unburnt carbon are thus given another chance to complete their combustion. xe2x80x9cLatexe2x80x9d air injection is provided in addition to the primary air injection performed at the bottom of the hearth, and in addition to at least one secondary air injection performed to create a weakly-oxidizing atmosphere in the hearth above its bottom zone in which a reducing atmosphere is created.
In a preferred implementation of the method of the invention, the late air injected upstream from the inlet of the cyclone is guided so as to modify the paths of the solid particles sucked up by cyclone action so as to increase the collection efficiency of the cyclone, and thus increase the combustion efficiency of the system.
In an implementation of the method of the invention, the late air injection of new and hot air is used to improve the combustion of the unburnt gases contained in the flue gases coming from the hearth by means of the additional mixing action that it causes.
In an implementation of the invention, the flow rate of the late air injected upstream from the inlet of the cyclone is chosen to lie in the range approximately 5% of the total air flow rate delivered to the system for combustion purposes to approximately 30% of said total air flow rate.
In an implementation of the method of the invention, the primary, secondary, and late air injections are such that they lead to excess air coefficients xcex that are respectively about 0.5 in a lower portion of the hearth in the zone in which the atmosphere is a reducing atmosphere, in the range 0.95 to 1.15 in an upper portion of the hearth, situated above the preceding portion, and in which the atmosphere is a weakly-oxidizing atmosphere, and from 1.15 to 1.3 beyond said upper portion, where the late injection produces its effects. It is thus possible to achieve a significant decrease in nitrogen oxide (NOx) emissions.
The invention also provides a combustion system of the circulating fluidized bed type, making it possible to implement the above-described method.
This system includes a hearth associated with at least one cyclone, in particular for delivering energy to a heat exchanger in an electricity-generating installation, and it operates using fuel that is inserted into the bottom of the hearth, where a reducing atmosphere is created and where the fuel undergoes pyrolysis with separation into two phases, namely a solid phase made up of grains of coke containing a nitrogen-coke fraction and coming from the original fuel, and a gaseous phase containing volatile matter and in particular the remaining nitrogen contained in the original fuel, the particles and the volatile matter rising up inside the hearth where combustion continues while leaving incompletely burnt particles which are sucked up and returned to the bottom of the hearth by the cyclone.
According to a characteristic of the invention, the combustion system includes means for performing xe2x80x9clatexe2x80x9d air injection for injecting late, new and hot air, which means are positioned between the top of the hearth and the inlet of the cyclone, and are organized to act on the paths of the sucked-up particles so as to increase the collection efficiency of the cyclone, means for performing primary air injection into the bottom of the hearth so as to create a reducing atmosphere there, and means for performing secondary air injection to create a weakly-oxidizing atmosphere in the zone of the hearth situated above its bottom zone in which a reducing atmosphere is created.
In the invention, the late air injection means comprise injectors opening out into a duct via which the top of the hearth is connected to the inlet of the cyclone, the injectors opening out into said duct via orifices situated in the duct inside wall, optionally as well as injectors opening out into the duct via orifices provided in the duct through its roof.
In a variant embodiment of the invention, the combustion system comprises late air injectors fed with hot and new air from an air heater of the system, via a circuit that delivers the secondary air for the system.
In another variant embodiment, additive injectors are positioned in late air injector orifices so as to penetrate into the duct via said orifices, and so as to facilitate additive penetration into the mass of flue gases into which the additive is injected.